The Present Invention relates to a cold deep hole forming device, especially a cold deep hole forming device used for forming, by the method of cold forging, a deep hole for an involute spline, etc., in a member to be formed partially different in thickness.
Conventionally, formation of a deep hole for an involute spline, etc., in a member to be formed partially different in thickness is made by supporting the member to be formed on a supporting base, lowering a punch for forming the deep hole on the member to be formed from above and press fitting the punch in a prepared hole roughly formed in advance in the member to be formed, thus forming a deep hole for the involute spline, etc. in a prescribed shape in the member to be formed.
A problem with the conventional cold deep hole forming device was that, since it is designed to press fit the punch in a prepared hole roughly formed in advance in the member to be formed, in a state in which the member to be formed is supported on a supporting base, it is impossible to form the deep hole etc . . . uniformly from an end face (inlet of the hole) to an inner part, e.g. of a member W to be formed as shown in FIG. 3 (B) (FIG. 3 (B), which indicates changes in BBD (Between Ball Diameter) from the end face to the inner part of the member W to be formed with an involute spline by the conventional cold deep hole forming device, shows that the involute spline formed is of poor accuracy, deviating from the upper and lower limit values of BBD, in the range of the effective length of the spline). This is especially the case where the object member W to be formed is partially different in thickness as shown in FIG. 1 and FIG. 2, thus making it impossible to obtain formed pieces of high accuracy.
On the other hand, efforts are being made to solve this problem with a method which consists in press fitting the punch in a prepared hole, roughly formed in advance in the member to be formed, in a state in which the member to be formed is inserted in a die and constrained at its outer circumference, so as to form a deep hole for involute spline, etc. in a prescribed shape in the member to be formed. However, this method also presented problems such as seizure accidents between the punch and the member to be formed, impossibility of extraction of the formed piece from inside the die, etc., because there is no way to control the restraining force on the outer circumference of the member to be formed to a proper value.